Oh also, are you using hub centric rings with those wheels? Not having any can cause issues like your describing

 

Driveshaft is a brand new shaftmasters driveshaft. IMO the old driveshaft it would vibrate around 75 mph and higher but with the new driveshaft it doesn't start till around 100 mph.

I've read that poly diff bushings have been known to cause this but on my previous Miata I had garage star delrin and never noticed any vibration. However the garage star bushings don't require the rubber to be separated from the mounting plates like the polys do. So there still is som compliance.